Bone fixture assembly

ABSTRACT

An assembly for rigid bone fixation includes a plate and a screw. The screw can be rotated freely within the plate in a non-locking configuration to secure the plate against the bone. A locking mechanism is engaged to prevent the screw from moving relative the plate. In one embodiment, the plate includes a threaded portion and a non-threaded portion. The screw head rotates within the non-threaded portion to tighten the screw and pull the plate against the bone. The locking mechanism can include a moveable nut that rotates within the threaded portion of the plate to lock the screw and plate in a locking mode. The assembly can be used for rigid fixation of bones that experience cyclic loads, such as the sternum and mandible. Methods of rigid bone fixation are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part application of U.S. application Ser. No.13/071,155 filed Mar. 24, 2011, which claims the benefit of U.S.Provisional Application No. 61/172,060 filed on Apr. 23, 2009 andInternational Application No. PCT/US2010/032269 filed Apr. 23, 2010, theentire contents of the above applications being incorporated herein byreference.

BACKGROUND OF THE INVENTION

Nearly 700,000 open heart surgeries are performed annually in NorthAmerica. During the surgery, the sternum is bisected to access thethoracic cavity, a procedure known as sternotomy. Following the primaryoperation, the sternum is closed, typically with wire sutures.

The sternal reapproximation procedure is generally successful. However,post operative complications occur in approximately 2% of procedures,generally in patients over the age of 65. High instances of osteoporosisthat are common in this age group causes the sternum to wear away atfixation points, causing loosening within the system. When looseningoccurs, other complications can arise, such as medianstinitis, orinfection of the sternum, which has been shown to have a mortality rateas great as 15%.

Low cyclic forces generated by repetitive motion, such as breathingmotion in the case of the sternum, can lead to unwanted loosening andeven failure of the fixation system. This problem becomes even moreacute in the case of osteoporotic or lower-density bones. Currently themost common practice of sternal fixation utilizes stainless steelsurgical wires. However, for some patients, the low cyclic forcesgenerated by breathing can cause the wires to cut into the patientresulting in losses of fixation and normal bone alignment. Rigidfixation techniques, such as with a plate and screw assembly, are known,but are not commonly used for sternum fixation. This is due to a varietyof factors, including the length of time for the procedure and increasedlevel of skill required by the surgeon, as well as the cost. Inaddition, existing plate and screw assemblies are not optimized forfixation of the sternum.

SUMMARY OF THE INVENTION

The present invention relates to a bone fixation device and morespecifically to sternum fixation with a screw and plate assembly. Thescrews pull the plate to the bone and have a locking capability. Nomicro-movement issues occur as with an unlocked beveled screw where theangle of the screw can be off axis and generate movement of the bonerelative to the plate.

A preferred embodiment enables higher torque which decreases looseningof the assembly over time. The screw/head interface minimizes wedgeangle effect. A preferred embodiment uses a higher number of threads forimproved purchase in cortical bone as well as increased depth of threadsfor improved purchase in cancellous bone. The locking mechanism inhibitssawing effect which can degrade the fixation given the repetitiveloading experienced by many bone structures during healing.

In one embodiment, the outer diameter of the screw is generally equal tothe plate hole inner diameter, which helps minimize initial loosening. Acap is used to secure the screw to the plate after the screws havesecured the plate into position against the bone surface.

The preferred embodiment further includes a method of securing fracturedbone elements together and/or repairing surgically separated boneelements in accordance with the devices and methods described herein.

A preferred embodiment can utilize a locking element in which the headof the screw utilizes a movable portion that is displaced by rotation ofthe head. Lateral movement of the movable portion of the head can beachieved by utilizing slots in the screw head that accommodate expansionas the screw is turned to engage external threads that force the screwin a longitudinal direction. This operates to increase contact pressurebetween the screw head and the contact region on the plate. Lateralmovement of the head displacement elements can be accomplished using asecond screw threaded within the first screw that has the expansionhead, or alternatively, the moveable portion of the head can engage aportion of the plate that is sufficiently rigid to displace the moveablehead as the screw is turned into a locked position. Another alternativeemploys a moveable element on the plate that engaged a static screw.

A further preferred embodiment relates to a method of manufacturing abone fixation device as set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will be apparentfrom the following detailed description of the invention, taken inconjunction with the accompanying drawings of which:

FIG. 1 is a side schematic illustration of a screw and plate assemblyaccording to one embodiment of the invention;

FIGS. 2A-2C schematically illustrate the locking of the screw to theplate;

FIGS. 3A and 3B are profile views of the bone fixation screw accordingto one embodiment;

FIGS. 4A and 4B are perspective views of the bone fixation screw;

FIG. 5 is a cross-sectional view of the plate;

FIG. 6 illustrates yet another embodiment of a screw and plate assemblyhaving a threaded plate and locking cap;

FIG. 7 illustrates a sternum closed by rigid plate fixation;

FIG. 8A illustrates a straight plate;

FIG. 8B illustrates an X-shaped plate;

FIG. 9 illustrates a friction wave plate with directional barbs;

FIG. 10 illustrates cortical and cancellous screws;

FIG. 11 illustrates the testing setup for cyclic load testing of sternumfixation devices;

FIG. 12 is a displacement curve resulting from cyclic loading of asternum fixation device;

FIG. 13 schematically illustrates the loading forces on a screw;

FIG. 14 illustrates the motion of a screw and plate having a wedgeinterface;

FIG. 15 schematically illustrates the effect of decreased screw headslope and resulting forces;

FIG. 16 illustrates screw movement due to plate inner diameter; and

FIG. 17 illustrates a screw and plate assembly with the screw diametermatching the plate hole diameter.

FIG. 18A illustrates a preferred embodiment of a locking screw assemblyin accordance with the invention.

FIG. 18B illustrates a perspective view of a locking screw assembly inaccordance with the invention.

FIG. 19A is a side view of a screw expansion head in accordance with apreferred embodiment of the invention.

FIG. 19B is a side cross-sectional view showing an expansion head with aplate displacer or expander in accordance with a preferred embodiment ofthe invention.

FIG. 19C is a perspective view of a plate with a plurality of expansionhead screws in accordance with preferred embodiments of the invention.

FIG. 20A is a perspective view with partially transparent plate featuresto illustrate screw well features in accordance with preferredembodiments of the invention.

FIG. 20B is an enlarged partial side view in which a displacer elementon the plate engages an opening in the screw head to displace a movablesection of the screw head in accordance with a preferred embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

This application claims the benefit of U.S. Provisional Application No.61/172,060, filed on Apr. 23, 2009, and also of InternationalApplication No. PCT/US2010/032269 filed Apr. 23, 2010, the entirecontents of the above application incorporated herein by reference.

Referring to FIGS. 1-5, an assembly 100 for rigid bone fixation, and inparticular, fixation of a sternum, includes a screw 101 and a plate 103.The plate 103 has a first surface 105 and a second surface 107 and anopening 109 extending through the plate 103. The screw 101 is a bonescrew having a head portion or flange 111 and a threaded portion 113. Ina bone fixation procedure, the second surface 107 of the plate 103 ispositioned adjacent to a bone portion 102. The threaded portion 113 ofthe screw 101 is inserted through the plate opening 109 and screwed intothe bone 102 to anchor the plate 103 to the bone. The first threadedportion 113, or screw shaft, can have a length of between about 0.4-1.4cm with an outer diameter of ˜4 mm, an inner diameter of ˜2 mm, and apreferred range of pitch of 1-1.25 mm. For other applications, such asmammalian leg bones, the mandible bone or other bone fixtureapplications, the pitch or distance between the threads (thread spacing)can be in a range of 0.8 mm to 1.6 mm. The depth of the threads can bein a range of 1.5 mm to 2.5 mm. In general, the plate 103 has aplurality of openings 109 (see, e.g., FIG. 8B), and a plurality ofscrews 101 are inserted through the openings 109 to anchor the plate 103to respective bone portions 102 in order to physically hold the boneportions together, limiting their movement and facilitating bonehealing. In one embodiment, the present plate and screw assembly 100 isused for rigid fixation of a human or animal sternum.

In one embodiment, shown in FIG. 1, the opening 109 in the plate 103 hasa first, larger diameter proximate the first surface 105 and a second,smaller diameter proximate the second surface 107. The threaded portion113 of the screw 101 extends through the smaller diameter portion of theopening 109 and into the bone portion 102. The head portion or flange111 of the screw is received within the larger diameter portion of theopening 109, but is prevented from passing completely through theopening 109 by a shoulder portion or plate retainer 115. The retainer115 has a planar surface that contacts a planar surface of the flange111.

The plate opening 109 has a threaded portion 117 and an unthreadedportion 119. The threaded portion 117 (e.g., tapped M8×1.24, “I” drill,0.272″) extends partially along the opening in the axial direction, andis generally adjacent to the first surface 105 of the plate 103. Theunthreaded portion 119 of the opening 109 extends between the threadedportion 117 and the shoulder 115.

The head portion 111 of the screw 101 is threaded (e.g., M8×1.25 Die, 8mm diameter), and threads into the threaded portion 117 of the plateopening 109. In operation, as shown in FIGS. 2A-2C, for example, thescrew 101 is screwed into the plate 103 using the matching threadedportions of the screw head 111 and the threaded portion 117 of the plate103 (FIG. 2A). The screw head 111 clears the threaded portion 117 of theplate opening 109 and enters into the unthreaded portion 119 of theopening 109 which provides a non-limiting free space (FIG. 2B). With nothread interactions between the screw 101 and plate 103, the screw 101can be fastened without plate restriction to increase the pressure ofthe plate 103 to the bone portion 102 (e.g., sternum). Here, the screw101 can be rotated as much as desired, and a friction fit can beachieved. At this point, the assembly 100 is non-locking.

The screw 101 can further include a locking mechanism. In the embodimentof FIGS. 1-5, for example, the screw 101 includes a moveable nut 121 onthe head portion 111. The nut 121 has the same outer diameter (e.g., 8mm) and external threading (e.g., M8×1.25 die) as the head portion 111.The nut 121 also includes internal threads (e.g., M3×0.5 tap, #39 tapdrill, 0.0995″), and is threaded to a threaded projection 123 (e.g.,M3×0.5 die, 3 mm diameter) that extends from the top of the screw 101,as shown in FIGS. 3A and 3B.

As shown in FIGS. 2A and 2B, the nut 121 is adjacent to and generallyflush with the threaded portion of the head 111 while both componentspass through the initial threaded portion 117 of the plate opening 109and arrive in the unthreaded free-space portion 119 of the plate opening109 as a single unit. The screw 101 can thus rotate limitlessly infree-space so long as the nut 121 and head 111 remain in thissingle-unit configuration. The torque feedback of the screw 101 is onlyfrom the bone 102 and is not influenced by the plate threads. The screwhead 111 engages the plate 103 at the shoulder 115 and the rotation ofthe screw 101 pulls the plate 103 into intimate contact with the bone102.

Once the screw 101 has reached the target torque and is ready to lock tothe plate 103, the nut 121 is deployed by rotating it relative to thethreaded projection 123 on the top of the screw 101. This rotation is inthe opposite direction from the direction of rotation during insertionof the screw into the plate. This rotation causes the nut 121 to “backup” (i.e., move up on the threaded projection 123) and re-engage withthe threaded portion 117 of the plate opening 109. This is illustratedin FIG. 2C.

In one embodiment, the exterior threads of the nut 121, which engagewith the plate threads, are different from the interior threads thatengage the nut with the projection 123 on the screw 101. The threads canhave slightly different pitch, for example, so that during the backingup of the nut 121, the difference in threads causes the nut to bind orlock up and provide backpressure to the screw. The assembly 100 is thenlocked. The locking can be accomplished within a half to a full turn ofthe nut 121.

The screw/plate assembly 100 allows a full friction fit in addition torestricting screw loosening in the plate.

To remove the screw 101, the nut 121 is rotated back down the threadedprojection 123 and onto the top of the head portion 111. This unlocksthe screw 101 from the plate 103. The entire screw 101 can then berotated in the reverse direction of FIG. 2A to remove the screw from thebone and plate.

As shown in FIGS. 4A and 4B, the nut 121 and threaded projection 123 caninclude respective grooves 125, 127 or other features to enable thescrew 101 and nut 121 to be rotated together during insertion andremoval of the screw 101, as well as for the nut 121 to be rotatedseparately for locking and unlocking of the screw to the plate.

In the embodiment of FIGS. 1-5, the interface between the screw 101 andplate 103 is substantially planar, though it will be understood thatalternative configurations, such as a beveled or rounded interface canbe employed. An advantage of certain embodiments is that the lockingmechanism restricts relative motion between the screw 101 and the plate103 in the axial direction, which prevents undesirable rotation of“wobbling” of the screw relative to the plate and minimizes oreliminates micro-motion issues.

In another embodiment, only the nut 121 includes external threads thatmate with the threaded portion 117 of the plate opening 109, and thehead portion 111 of the screw 101 is unthreaded. This embodimentoperates substantially as described above in connection with FIGS. 1-5.

FIG. 6 illustrates yet another embodiment of a screw and plate assembly150 having a threaded plate 153 and locking cap 155. In this embodiment,the screw 151 is not threaded on the head portion of the screw. Thescrew 151 is inserted through the plate hole and tightened, with thescrew head being received by a beveled shoulder in the plate 153. Oncethe screw 151 is sufficiently tightened, a locking cap 155 havingexternal threads is threaded onto the plate 153. The cap 155 preventsthe screw 151 from rotating or wobbling relative to the plate 153.

The screw and plate can be made from any suitable biocompatiblematerials. In general, the materials have high-strength and aregenerally rigid, although some flexibility and compliance may bebeneficial. Suitable materials include, for example, titanium andstainless steel.

The present invention is directed to a method of rigid fixation of asternum. The anatomy of an adult human sternum 200 is shown in FIG. 7.The sternum is also known as the breastbone and occupies the centralanterior thorax. In conjunction with the first seven pairs of ribs, itencapsulates the heart and lungs. The average length of the sternum inadults is 17 cm. Because the sternum encloses the lungs, it must becapable of flexing during inhalation and expiration. Thus, the sternumcontains a high percentage of spongy trabercular cancellous bone, with athin cover shell of dense, compact cortical bone.

In a sternotomy procedure, the sternum is bisected longitudinally alongits center and retracted to access the thoracic cavity. Once the primaryoperation is complete, the surgeon follows with sternal fixation, inwhich the halves of the sternum are fixed together so that the bone willheal properly. In one embodiment, a method of rigid sternum fixationuses a plate and screw assembly such as shown and described inconnection with FIGS. 1-6. An example of a sternum 200 closed by rigidplate fixation is illustrated in FIG. 7. A plurality of plates 103 areaffixed to the sternum 200 using screws 101 and hold the two halves 200a, 200 b of the sternum in place while it heals. Each plate 103 is fixedto the bone using a plurality of screws 101 inserted through holes inthe plate. Generally, at least three and preferably four or more screws,distributed on either side of the sternum bisection, are utilized toattach the plate. Rigid plate fixation offers advantages over othertechniques, such as wire fixation, because it physically holds the boneportions together during the healing process, limiting their relativemovement and not disrupting the blood supply in the region.

Plates are usually designed and manufactured specifically for a clinicalapplication. Due to the large variety of bone shapes and sizes withinthe body, there are several different types of rigid fixation platesthat can be used. For example, there are straight plates, X-shapedplates, wave plates, and friction plates. FIGS. 8A and 8B illustrateexemplary embodiments of a straight (FIG. 8A) and an X-shaped (FIG. 8B)plate.

X-shaped plates may be an effective option for sternal fixation due tothe fact that they enable multiple screws to pass through the center ofthe bone.

Wave plates are a variation of the straight fixation plate, and arewidely used in long bone compression fixation. These plates arebeneficial because they do not apply compressive forces directly to thefracture site. Applying extensive compressive forces to the wound sitecan increase vascular disruption to the wound site, limiting the bloodsupply to the wound site and increasing healing time. Although thisdesign is usually applied to large cortical bones, it can be useful fordecreasing the healing time of a very vascular bone such as the sternum.

The friction (or adhesive) plate system adds ridges to the undersurfaceof the plate, increasing the plate-bone contact area and effectivelydecreasing the stresses on the screws used in the system by as much as athird. These plates come in a variety of shapes and sizes, and can beadded to almost any plate and screw system, creating a very versatilesystem. By minimizing the motion between the plate and the bone, stressprotection in that region of bone can be greatly reduced. Although thissystem has not been widely accepted, it can be very advantageous forsternal fixation, since the screws that are used are much smaller andhave much more cyclic loading than plate systems elsewhere in the body.

In one embodiment, a rigid fixation assembly includes a friction waveplate 300 as schematically illustrated in FIG. 9. The plate 300 hasdirectional barbs 303 that distribute the lateral loads over theentirety of the plate to bone surface. By including these small anchors,the press fit may be slightly relaxed to improve bone vascularity. Theconcern of a locking system leveraging is reduced since the plate doesnot solely rely on one point of purchase, but on many distributedpoints. The center break where the sternum halves reunite include aslight wave 305 to guarantee no pressure is compromising the healingfactors.

Rigid bone fixation is possible mainly due to a large variety of bonescrews, and over the past 20-30 years the bone screw has become the mostcommonly used orthopedic implant device. Without these screws, manytypes of rigid fixation would be much less effective or even impossible.Each type of screw is uniquely designed for its specific clinicalpurpose. Several parameters are taken into consideration when choosing ascrew, including the health of the bone at the wound site (osteoporoticor healthy), the location of the fracture (long bone, short bone, flatbone etc), the density of the bone (cortical or cancellous) and theytype of fracture. A majority of orthopedic bone screws are categorizedas cortical or cancellous, partially or fully threaded, solid orcannulated, self-tapping or non-self-tapping.

The cortical or cancellous properties of the screw are determined basedon the density of the bone that the screw is being applied to. Corticalscrews are very similar to metal screws found in a local hardwarestore—they have a very high thread count, with a very low thread depthand pitch. Because they are used in the hardest, highest density type ofbone, thread penetration is not very important, but it is vital that thethreads stay in constant contact with the bone surrounding it.Conversely cancellous screws are very similar to wood screws, boastingdeeper thread penetration to maximize stabilization in the low densitycancellous bone.

Stabilization of an implant or plate is greatly dependant on thebone-screw interface. The screws in a rigid fixation system function asstabilizers by exerting a compressive force on the plate and onto thebone. The screws also provide resistance to shear forces when the plateis loaded axially. The different parts of the screw (e.g., head, coreand thread) serve to achieve the functions of providing compressiveforce and maintaining purchase in the bone material. The head of thescrew functions to transmit the insertion torque to the core and threadsand to provide a stop when the head contacts the bone surface. Once thescrew head has contacted the bone, the torque exerted on the threadsthrough the head generates a compressive force. In locking platesystems, the head is also threaded such that it locks into respectivethreads on the plate. This system limits the torque to which the screwcan be tightened as well as prevents wobble of the screw.

The core is the screw shaft that the threads wrap around. A screw isdefined by a major diameter that is measured from the outside of thethreads on one side to the outside of the threads on the other as wellas a minor diameter that defines the smallest diameter of the shaft atthe base of the threads that represents the core. The length of the coreis of particular interest in its application to the sternum. Screws canbe categorized as unicortical or bicortical (in which the end of thescrew is embedded into the innermost cortical layer). Bicorticalinsertion can provide greater stability with respect to both wobble andpullout, but can also cause damage to the bone and to tissue and/ororgans beyond the bone.

The screw thread is defined by its depth (difference between the majorand minor diameter) and its pitch. The thread depth is what responsiblefor thread purchase as it represents the area of the screw that isinteracting with the bone. The thread is a helical ridge that is wrappedaround the core. Its function is to convert rotation into translationalmovement. The cross section is a series of ramps. Together with thehelical shape, when rotated the triangular cross section functions as aninclined plane that provides a mechanical advantage in moving throughthe bone and to maintain a compressive force. The thread pitch isdefined as the distance between threads on the screw.

Screw anatomy can be modified to work most effectively in differenttypes of bone. The use of screws in bone fixation has directed screwdesign to either cortical or cancellous application. Cortical screws aredesigned for purchase in dense bone with shallow threads cut at about60° and decreased pitch. Cancellous screws typically follow a woodscrew's design that includes a tapered outside diameter for easierinsertion and wider threads to increase purchase in less dense andcompressive bone. Each of these screw types can be seen in FIG. 10.

When coupled with a plate, the screw design becomes more complicated.For internal fixation of fractures, the plate and screw system shouldhold any force that would normally be applied to the bone whilemaintaining its position. This loading can happen in two ways, either bythe frictional contact between the plate and the bone or the interactionbetween the shaft of the screw and the bone. If the primary mechanism ofloading relies on the screws, than the contact interface between theplate and the shaft of the screw may cause the screw to bend in cyclicloading. Screw bending will rapidly result in the early fatigue failureof the screw. Because of this, the preferred mechanism of loading is forthe plate to be friction fit with the cortical bone.

Rigid fixation methods are used throughout the body, however aspreviously discussed, they are not commonly practiced on sternum. Thereare many screw plate systems each designed to accommodate a specificbone's aspect, such as pelvic plating systems, which allow screws to beinstalled at various angles to maximize rigidity. However, the sternumis different from other bones due to the continuous applied loads frombreathing. Additionally, the sternum cannot be voluntarily immobilizedduring the recovery period. In order to identify the effective optionsfor rigid sternal fixation, the mechanisms of loosening and failure dueto lateral cyclic loads can be investigated.

Cyclic testing was performed to compare the performance of commerciallyavailable screw types for rigid sternum fixation. Testing was alsoperformed using an embodiment of a present plate/screw assembly thatallowed locking of the screw after a complete friction fit is achieved.

Four non-fixed human sternums with varying degrees of osteoporosis werebisected lengthwise and cut laterally into strips. Bone fixation plateswere attached to the samples and cyclically loaded. Cyclic testing wasperformed using an Electroplus E-1000 uniaxial mechanical test device(Instron®). Testing consisted of cycling between 0 and 50 N at a rate of2 Hz for 15,000 cycles. The bone pieces were potted in a PVC cap withepoxy. A custom grip was used to hold and vertically load the plate.Local distraction between the plate was measured using an extensometer.

Cancellous and cortical pelvic screws and pedicle screws with lockingand non-locking heads provided by Stryker® were cyclically tested. Anexample of the present plate/screw assembly (Embodiment A) was alsotested. The screws were paired and tested down the length of thesternum. FIG. 11 illustrates the testing setup.

MicroCT measurements with a resolution of 15 μm were made on each sampleto determine the local bone density and to view the mechanism ofloosening within the bone.

The initial (10 cycle) and final (15,000 cycle) displacement data wereanalyzed statistically using a two-way ANOVA with screw type andbircortal or unicortal purchase as the factors (p<0.05 consideredsignificant).

Most of the loosening was observed in the first 10 cycles. Afterapproximately 2000 cycles, where is a slow increase in displacement thatcontinues for the next 13000 cycles. A typical displacement curveresulting from cyclic loading is shown in FIG. 11.

In the initial 10 cycles, the displacement of screw types were notsignificantly different. Though unicortical attachment had asignificantly larger displacement compared to bicortical (1.05 mm vs.0.36 mm, p=0.015). The screw type appears to affect the finaldisplacement following the 15,000 cycles more so than the number ofcortices anchored. The mean data of groups is expressed in Table 1.

TABLE 1 Summary of displacement data (mm, mean ± SD) UnicorticalPurchase Pelvic Pedicle Pedicle Pelvic Embodiment Cancellous LockingCortical Cortical A Initial 1.43 ± 0.261 ± 0.534 ± 1.01 ± 0.183 ± 0.4110.022 0.501 0.555 0.084 Final 2.69 ±  1.54 ±  1.57 ± 1.15 ± 0.458 ±0.104 0.69 1.23 0.721 0.182 Bicortical Purchase Pelvic Pedicle PelvicCancellous Cortical Cordical Initial 0.421 ± 0.216 ± 0.256 ± 0.102 0.1290.127 Final 3.31 ± 0.355 ± 0.625 ± 1.65 0.313 0.584

The final mean displacement of cancellous screws was significantlylarger than cortical (2.60 mm vs. 1.17 mm, p=0.039). The Embodiment Ascrew outperformed the others, often with a lower final displacement(after 15,000 cycles) than the initial displacement of the other groups(at 10 cycles).

The findings indicate that both screw type and cortical purchases areimportant parameters for a screw-plate system for sternal closure inosteoporotic patients. Though the sternum is mostly cancellous bone,osteoporotic cancellous bone is generally too weak for adequatefixation. Pullout test within the cancellous region of the sternumquantified the low strength of this bone. Cortical screws demonstrategreater resistance to transverse loading due to additional threadpurchase into the cortical layers.

Bicortical purchase minimized displacement during cyclic loading by twomechanisms. First, the screw is loaded against two relatively rigidstructures changing the loading from a singe to double shear model,limiting screw wobbling and bone damage. Second, the total amount ofthread purchase in cortical bone increases.

However, bicortial purchase may not be clinically recommended due to therisk of injuring the heart. Unicortical locking screws can be used torestrict screw wobbling, yet once secured to the plate it cannot befurther tightened it cannot be further tightened to the bone thuslimiting the friction fit. Unicortical non-locking screws provide afriction fit, but are susceptible to wobbling and early loosening.

The plate/screw assembly of one embodiment of the present inventionadvantageously utilizes a non-locking screw with a post-lockingmechanism that changes between a non-locking and locking mode. Asdiscussed above, the screw is fastened into a partially threaded platepast the threaded portion of the hole and into the threadless freespace. At this time with no thread interactions, the system isnon-locking and the screw may be fastened without restriction to achievefriction fit. When the screw is ready to be locked in the plate, alocking mechanism is engaged. This system satisfies the goal of allowinga full friction fit in addition to restricting screw loosening in theplate.

In one embodiment, the present screw includes a cortical pitch withincreased thread depth. A screw design specifically for the sternum,which is composed of cancellous bone between two layers of corticalbone, includes screw parameters designed for both cortical andcancellous bone types. It is important in cancellous bone to have asmuch bone as possible captured between the screw threads for maximumpurchase as larger volumes of bone surrounding the screw increasesresistance to backout. Screw parameters such as thread depth, shape, andpitch can be designed for specific types of cancellous bone to optimizethis volume.

The loosening of screws is due to two types of forces, as shown in FIG.13. Vertical forces result in pullout of the screws while horizontal, oraxial, loading forces, results in loosening through force passed throughthe plate, which places stress on the bone, cutting it over cyclicloading.

The rigid sternal fixation design is comprised of a plate and screwinterface. Multiple screws anchor the rigid plate against the sternumhalves. The design consists of three interfaces that influencemechanical loading: screw to plate, screw to bone, and plate to bone.The design alternatives are based off the design criteria and themechanisms of loosening identified from the tests.

The screw to plate interface is concerned with the degree of freedom thescrew is permitted after installation. A conventional locking screw andplate limits the pressure of the plate to the bone due to prematurelocking. If the plate is floating atop and not securely pressed againstthe bone, the cyclic forces apply additional leverage against the bone.By using a non-locking mechanism the plate can be pressed against thebone as much as the screw can be tightened. However a screw that locksinto the plate remains rigid.

The Stryker pelvis plate systems, for example, were designed to allowthe screw to freely pivot approximately 30° within the plate.Experimental results indicate this pivoting mechanism provides poorrigid fixation. The screw is designed to be loaded traverse, however ifthe screw is pivoted to a certain extent the screw is loaded similar toa pullout. As the screw pivots the softer cancellous bone becomes gougedand destroyed. Screws that maintain a permanent angle with the platedistribute the cyclic forces evenly throughout both the cortical andcancellous bone.

Accordingly, the screw to plate interface should keep the screw at afixed angle in the plate. One option to achieve this is to use a thickerplate with a fitted shoulder directing the screw perpendicular to theplate. Another design incorporates an additional locking cap component.A non-locking screw is installed followed by a threaded cap thatprovides direct parallel pressure to the cap of the screw preventing anypivoting or motion once. A third design is similar to the second howeverrather than an additional screw on cap, a thin rigid bar is slid intoplace.

The results indicate cortical threads minimize displacement better thancancellous threads. Even though the sternum is composed more ofcancellous than cortical bone, the cancellous region being osteoporoticdoes not exhibit significant structural integrity. The screw design isprimarily focused on achieving the greatest fixation in the corticalbone layer. The cancellous screws had an insufficient number of threadsin the cortical bone; a high thread density screw permits greater threadsurface to cortical bone. The screw threads are the primary fixationsource of the entire plate system and must securely bite into thecortical bone.

The thread design should maximize cortical surface contact, generallywith only one cortex, and make do with the spongy cancellous region. Onescrew design uses a high thread density with a smaller outer diameter atthe tip. The smaller diameter is intended to lightly anchor into thesecond cortical layer, while the remaining screw is wider and fixates tothe remaining bone. Another design maintains a constant outer diametersize with a decreasing inner diameter, in which the screw core tapers inthe distal direction of the screw cap. A third design has a lower threaddensity to offer enough distance for the threads to curve slightlybackwards, acting as barbs clinching against the bone. Yet anotherdesign uses a corkscrew as a means of gripping into the bone as opposedto threads.

The interface between the plate and the bone consists of the surfacearea contact when the plate is compressed against the bone. In theeffort to minimize local distraction between the bone halves a platethat resists shear loading against the bone is desirable. In certainembodiments, this can be achieved through use of a friction plate. Inthe specific case of a sternotomy, the periosteum covering the sternumis not usually removed, resulting in indirect contact of the plate andthe bone. The plate to bone tightness generally depends on the fixationof the screw, however a higher friction coefficient reduces the platesliding on the periosteum of the sternum option proposed having a platewith similar roughness to a filer.

Excessive pressure against the bone from another surface may increase avascularity causing osteonecrosis. This interface has clashingconstraints with the need for press-fit to prevent screw plateleveraging while simultaneously ensuring the bone properly heals.

Results of axial testing have shown that screw head design is animportant parameter in system loosening. The screw head designdetermines the interaction between the screw and the plate. For example,a screw head having a rounded or beveled bottom surface is able torotate or pivot freely in the plate, as seen in FIG. 14. Instead ofmoving the plate and screw together, only the screw is loaded. Due to awedge-type action between the screw and the plate, the screw is forcedaway from the bone. Axial loading then pull on the screw in a pull-outmechanism instead of lateral loading. The smaller the wedge angle, theless wedge leverage is available to pull the screw out, as seen in FIG.15.

Another design parameter observed to have an effect on loosening,particularly loosening due to initial loading, is the difference betweenthe inner diameter of the screw and the inner diameter of the plate.Typically, the minimum inner diameter of the plate hole is the outerdiameter of the screw so that the screw can pass through. The screw iscentered by the contour of the screw head to the plate. If the threadsare deep, the inner diameter of the screw is much smaller than thediameter of the plate hole. During axial loading, the screw will bepulled axially with immediate displacement occurring to the gap betweenthe centered screw and the edge of the plate (FIG. 16).

It is therefore concluded that a mechanism that keeps the screwgenerally perpendicular to the plate is desirable. This mechanism can beprovided by a locking screw design; however, the major limitation oflocking screws is the inability to control the amount of torque appliedto the screw and the purchase into the bone as the torque will belimited by the locking mechanism in the screw head. The locking effectcan be achieved by making the inner diameter of the screw as large asthe diameter of the plate (FIG. 17). A screw/plate system is proposed inwhich the plate is threaded to allow the screw threads to pass through.There is a section of the screw right before the screw head that doesnot have threads but is as wide as the thread diameter, with the conceptof having the inner diameter of the screw be as close as possible to theinner diameter of the plate.

The advantages to bicortical purchase include greater ability to tightenthe screw to higher torques providing higher plate compressive forcesand greater resistance to axial loading. However, in the application ofrigid fixation of the sternum, there is a great preference not to usebicortical purchase due to proximity of the heart. There are alsoproblems that arise when trying to use a screw bicortically inosteoporotic bone. Due to the lack of support of cancellous bone in anosteoporotic sternum that only weakly holds the cortical layerstogether, the second cortical layer can either be drawn upwards(compressing the sternum) or pushed downwards (expanding the sternum) bythe screw, thus damaging the bone. If screws are to be usedbicortically, then predrilling must also be bicortical, increasing riskof injury to the heart. Therefore, the ability to minimize localdistraction with only unicortical purchase is greatly beneficial.

Much of the quantitative data and observed mechanisms of loosening wereunexpected. Cortical screws proved to be more resilient against cycliclateral loading than cancellous even though the sternum is largelystructured form trabecular cancellous bone. Though bicortical isgenerally not an acceptable practice and may cause profound damage tothe cancellous region, limiting screw pivoting appears to benefit rigidfixation.

There is still concern of poor press fit from locking screw due to earlyunwanted locking. However, the non-locking screw results suggest theneed for a screw to remain fixed in its complementing plate to limit thescrew from pivoting in the plate and loosening. Non-locking screws areable to achieve full press-fit, however, are susceptible to screwpivoting leading to loss of fixation.

There were two concerns of wobbling: non-locking screws pivot within theplate whereas locking screws do not pivot in the plate but can lever theentire plate. The present antiwobble screw-plate assembly combined theneeded press-fit to securely fasten the plate against the bone and alocking mechanism to prevent the screw for pivoting in the plate.Testing demonstrated that the combination of full press-fit and lockingof the screw to plate significantly reduced the dehiscence produced bylateral cyctic loads more than these each of these mechanisms could doindependently.

One embodiment of the present backout-locking system combines the threaddensity of a cortical screw with cancellous thread blades to maximizecontact with the cortical bone layer. The screw head incorporates theantiwobble concept and is designed to toggle between a non-locking andlocking mode within the plate compartment. This allows for the user tofasten the screw without any plate restriction to achieve full press-fitand to follow-up with locking the head into the plate to preventpivoting. The overall findings and observations indicate the best optionof rigid plate fixation for osteoporotic sternum is the backout-lockingscrew-plate system.

A further preferred embodiment of the invention utilizes a two piecescrew assembly, sternal fixation plates and requires only one tool forsurgical installation. The screw is also unicortical and of both thecompression and locking type. The two part design is a screw within ascrew and was designed to optimize two important functionalconsiderations: ease of use by the surgeon and manufacturability.

The outer screw functions as a compression unicortical screw with ascrew head-plate interface. As seen in FIG. 18A, the outer screw has acountersunk head with radial expansion slits around its circumference.The bottom of the countersunk hole has receiving threads for theinternal screw. A unicortical thread pattern on the distal boneinterfacing screw body is employed as well as a self-tapping tip. Thebottom of the outer screw head can have a roughened surface that willinterface with the bottom of the plate welling.

The inner screw mates into the top of the outer screw with its wedgepitch being the same angle as the outer screw's receiving countersink,but the wedge length will extend past the top of the outer screw. Astandard Phillips head surgical tool can be used to interface with topof the internal screw.

When the threaded shaft of the inner screw is mated with the femalethreads on the inside of the outer screw, and gently tightened, it ispulled down into the body of the outer screw. This is the state of thedevice at which the surgeon will receive it. The screw is then insertedinto the plate and bone through normal surgical procedure. When thescrew has reached a tightness determined by the surgeon for compressionfit of the plate to the bone, the surgeon exerts a downward force toengage the roughened outer screw head's bottom surface with the fixationplate's well surface. This creates enough of a frictional force to allowthe inner screw to continue to rotate into the outer screw internal body(and without stripping the bone), thus creating an outward wedging forceon the outer screw head. The expansion slits allow the head of the outerscrew to expand into the walls of the plate, causing a force inducedlocking mechanism that holds the screw tightly at the plate interface.

As seen in FIG. 18A, a stainless steel flat head screw was used as theinner screw 406, which pushes against a modified stainless steel sockethead cap screw 402. As seen in FIG. 18B, expansion slots 420 in anexpansion portion 404 of the screw allow the user to insert the threadedportion 405 through a plate and into bone as described previouslyherein. The inner screw 406 in then tightened by the surgeon using notch416 to drive the threaded portion 408 further into the inner cavity 410of the screw 402. Surface 412 of screw 406 imparts a lateral force tomove 414 the expansion portion in a lateral direction to frictionallyengage the plate.

A screw-plate system manufactured where the outer screw can successfullybe press-fit and locked into the stainless steel plate. As the innerscrew is tightened, the outer screw splays out and frictionally engagesthe stainless steel plate, which prevents micro motion of the screw headwith the plate. This reduces the wobbling effect and minimize screwloosening against the bone.

A preferred method of manufacture involves using 2.3 mm non-lockingunicortical bone screw to act as the outer screw with a smaller 1.75 mmstandard screw. In order to manufacture such a small system, wireelectrical discharge machining is performed to create small cuts in thehead of the outer screw. Furthermore, the outer screw will be tappedwith a thread mill to create a threaded hole for the inner screw to beinserted. The standard bone plate will not be modified, as the outerscrew will expand and press-fit against the plate.

The effectiveness of the designed screw plate system has been determinedby comparing it to unicortical non-locking screw plate systems. Bothsystems were cyclically loaded to mimic breathing of a normal humanbeing to assess the amount of screw displacement.

In another preferred embodiment of the invention, the screw head canemploy movable sections or displacement elements that engage displacerelements on the plate to provide a locking mechanism. As seen in FIG.19A, a screw 500 has a head with slots 510 that define movable elements502, 504, 506 that can be displaced in lateral directions (508, 512, forexample). The screw 500 can frictionally engage the plate 540 as shownin FIG. 19B where a displacer element 546 on the plate operates todisplace a movable section 502. The displacer element or elements 546can protrude from the bottom surface of the wells 542. The well 542 inthe plate 540, as shown in FIG. 19C, can have a larger radius expansionwell feature 544 that accommodates the movement of elements 502, 504,506. As shown in FIG. 20A, the threads on the screw engage a threadedinterior lower bore 548 in the plate 540. The slots 510 in the screwhead, in combination with openings 545 situated in the bottom 547 of thescrew head that receive displacer element 546, accommodate the movementof the elements 502, 504, 506 when sufficient force develops between theplate displacer element 546 and the movable elements. The plate and thescrews can be metal or other materials with sufficient hardness andsurface friction to lock the components together. Note that a furtherembodiment can employ flexible or displacement features on the plate toprovide sufficient friction between the plate and the screw.

While the invention has been described in connection with specificmethods and apparatus, those skilled in the art will recognize otherequivalents to the specific embodiments herein. It is to be understoodthat the description is by way of example and not as a limitation to thescope of the invention and these equivalents are intended to beencompassed by the claims set forth below.

What is claimed is:
 1. A bone fixation device comprising: a plate havinga plurality of holes, at least one of the plurality of holes having athreaded interior; a plurality of threaded screws to attach the plate toa bone, each screw having a threaded portion sized to engage the atleast one hole having the threaded interior, a head with a displaceablehead portion and a slot in the head; and a screw locking displacerelement that extends from a surface of the plate to engage the slot inthe head to thereby displace a portion of the head relative to the screwto attach at least one of the plurality of threaded screws to the plate.2. The device of claim 1 wherein the screw has a plurality of movablehead portions.
 3. The device of claim 1 wherein each hole comprises afirst threaded section, a wider diameter nonthreaded section and anarrow diameter section.
 4. The device of claim 1 wherein each threadedscrew has a threaded length in a range of 0.4 to 1.4 cm.
 5. The deviceof claim 1 wherein the plate has at least three holes, each hole havinga helical ridge forming the threaded interior that engages a threadedscrew with a plurality of rotations.
 6. The device of claim 1 whereineach threaded screw has a thread spacing in a range of 0.8 to 1.6 mm. 7.The device of claim 1 wherein each threaded screw has cortical bonethread.
 8. The device of claim 1 wherein the threads have a depth in arange of 1.5 to 2.5 mm.
 9. The device of claim 1 wherein the platecomprises at least two holes on a first side to attach to a sternum boneon a first side of a patient's sternum and at least two holes on asecond side of the plate to attach to a second side of the patient'ssternum.
 10. The device of claim 1 wherein the plate further comprises aplurality of barbs.
 11. The device of claim 1 wherein the screwthreading has a length to engage a first cortical layer and secondcortical layer.
 12. The device of claim 1 further comprising at leastone of the threaded screws has threading to engage a cortical layer andcancellous bone layer.
 13. A bone fixation device comprising: a platehaving a plurality of displacer elements, the plate further including aplurality of threaded holes; and a plurality of threaded screws toattach the plate to a bone wherein each screw has a threaded portionsized to engage the threaded holes in the plate, at least one of thethreaded screws having a head, a movable element attached to the headthat engages one of the displacer elements on the plate to rigidlyengage the plate wherein the movable element is displaced relative tothe head upon engaging at least one of the displacer elements to attachthe screw to the plate.
 14. The device of claim 13 wherein each screwhas a plurality of movable head elements.
 15. The device of claim 13wherein each hole comprises a first threaded section, a wider diameternonthreaded section and a narrow diameter section to receive the head ofone of the screws.
 16. The device of claim 13 wherein each screwcomprises a plurality of slots in the head.
 17. The device of claim 13wherein at least one of the displacer elements is positioned in a wellin the plate.
 18. The device of claim 13 wherein the head of the atleast one threaded screw has an opening to receive the displacerelement.
 19. The device of claim 13 wherein the plate further comprisesa plurality of barbs that engage bone on one side of the plate.